# proposed fan and pump power

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Hi All,
I was wondering if anyone would be able to clarify how you model your proposed fan and pump powers. Do you typically use the nameplate motor HP or do you use the total pressure along with the mechanical and motor efficiencies. Using the nameplate motor HP to get a W/cfm or W/gpm seems to overestimate the fan/pump energy because the next available motor size is chosen (or the motor is oversized). The baseline on the other hand only uses the BHP with the motor efficiency for fan power and a straight W/gpm number for pumps.

On another note related to fan power, are laboratory exhaust fans (for general and fume hood exhaust) considered part of the "System Fan Power" or are the exhaust fan powers modeled so that the baseline and proposed are kept the same. The exhausted air is made-up via the air handling unit which also serves other non-laboratory spaces and has a return fan. I've read some discussions on this forum leaning towards including it as part of total system fan power but I've also seen LEED review comments stating that independent exhaust fans should be modeled identically between baseline and proposed models. It seems like keeping the exhaust fan powers the same in both cases helps boost the energy cost savings as the fan power allowance when excluding exhaust fan is distributed to only the supply/return fans.

Last question is do you break up your baseline total system fan power into supply/return/exhaust using the power or flow fraction of each applicable fan in the proposed design? I typically use the flow fraction but I was checking out the "Energy Model Input Translator" put out by RMI and it seems like they calculate baseline fan power by distributing the total system fan power by power fractions for each fan.

Example:
10,000 cfm supply: 0.0013*10,000 = 13 BHP
HP = 13/.91 = 14.3 HP

cfm

% flow

HP

Supply Flow

10000

0.5

7.1

Return Flow

8000

0.4

5.7

Exhaust Flow

2000

0.1

1.4

Total

20000

1

14.3

cfm

% flow

HP

Supply Flow

10000

0.56

7.9

Return Flow

8000

0.44

6.3

No exhaust fan

Total

18000

1

14.3

Lab system with 100% OA

cfm

% flow

HP

Supply Flow

10000

0.50

7.1

no return fan

Exhaust Flow

10000

0.5

7.1

Total

20000

1

14.3

A lab system with 100% OA could also just be 14.3 HP supply and the exhaust would be the same as the design exhaust power depending on how you treat the exhaust fan.

Any light shed on this would be very helpful. Thanks in advance.

Mirza

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Joined: 2011-11-16
Reputation: 0

Mirza --

I'll respond on one of those points.

On the issue of basic system fan power, proposed vs. baseline, I use the
installed motor horsepower for proposed and the calculated Appendix G kW/CFM
for the baseline. This overstates the energy consumption on the proposed
model somewhat but the approach is at least conservative, defensible and
simple. Even if the duct system has lots of restrictions and the filters are
dirty, you know that the real world energy consumption will be no worse than
the motor horsepower. Using this method I've found that the Appendix G fan
energy still gives me savings, even though I'm (likely) overstating the
proposed consumption. My take is that fan manufacturers are pretty good at
matching motor HP and performance to keep equipment costs down.

Steve Samenski, PE, LEED AP BD+C, BEMP

From: Mirza Sajjal Date: Tue, 15 Nov 2011 19:42:51 -0500
To: "equest-users at lists.onebuilding.org"
Subject: [Equest-users] proposed fan and pump power

Hi All,
I was wondering if anyone would be able to clarify how you model your
proposed fan and pump powers. Do you typically use the nameplate motor HP or
do you use the total pressure along with the mechanical and motor
efficiencies. Using the nameplate motor HP to get a W/cfm or W/gpm seems to
overestimate the fan/pump energy because the next available motor size is
chosen (or the motor is oversized). The baseline on the other hand only uses
the BHP with the motor efficiency for fan power and a straight W/gpm number
for pumps.

On another note related to fan power, are laboratory exhaust fans (for
general and fume hood exhaust) considered part of the ?System Fan Power? or
are the exhaust fan powers modeled so that the baseline and proposed are
kept the same. The exhausted air is made-up via the air handling unit which
also serves other non-laboratory spaces and has a return fan. I?ve read
some discussions on this forum leaning towards including it as part of total
system fan power but I?ve also seen LEED review comments stating that
independent exhaust fans should be modeled identically between baseline and
proposed models. It seems like keeping the exhaust fan powers the same in
both cases helps boost the energy cost savings as the fan power allowance
when excluding exhaust fan is distributed to only the supply/return fans.

Last question is do you break up your baseline total system fan power into
supply/return/exhaust using the power or flow fraction of each applicable
fan in the proposed design? I typically use the flow fraction but I was
checking out the ?Energy Model Input Translator? put out by RMI and it seems
like they calculate baseline fan power by distributing the total system fan
power by power fractions for each fan.

Example:
10,000 cfm supply: 0.0013*10,000 = 13 BHP
HP = 13/.91 = 14.3 HP

cfm% flowHP
Supply Flow100000.57.1
Return Flow80000.45.7
Exhaust Flow20000.11.4
Total20000114.3

cfm% flowHP
Supply Flow100000.567.9
Return Flow80000.446.3
No exhaust fan
Total18000114.3

Lab system with 100% OA
cfm% flowHP
Supply Flow100000.507.1
no return fan
Exhaust Flow100000.57.1
Total20000114.3

A lab system with 100% OA could also just be 14.3 HP supply and the exhaust
would be the same as the design exhaust power depending on how you treat the
exhaust fan.

Any light shed on this would be very helpful. Thanks in advance.

Mirza

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Joined: 2011-09-30
Reputation: 0